The invention relates to a rotary grinding mill for size reduction of particles such as ceramics, minerals and pharmaceuticals.
Prior art rotary mills include a cylindrical drum rotated about a generally horizontal axis. The rotating drum is fed with particulate material such as a slurry or powder, the rotation of the drum being at one half to three quarters of the xe2x80x9ccritical speedxe2x80x9d (i.e. the minimum speed at which material at the inner surface of the drum travels right around in contact with the mill). This causes a tumbling action as the feed and any grinding media travels part way up the inner wall of the drum then falls away to impact or grind against other particles in the feed. Size reduction of the particles is thus achieved principally by abrasion and impact.
In conventional rotary mills, the energy requirements of the mill increases steeply with increasing fineness of grind. For applications where a fine grind is required, the use of stirred mills, in which a body of the particulate material is stirred to create shearing of particles and numerous low energy impacts, may be used to ameliorate this problem to some extent. However, the present application of stirred mills is constrained by reduction ratio boundaries imposed by both upper feed size limits and energy transfer inefficiencies at ultra fine sizes. These constraints, together with throughput limitations and media/product separation difficulties due to viscosity effects at ultra fine sizes, restricts the practical and economic scope for applying that technology.
The present invention aims to provide an alternative grinding mill construction.
The invention, in one form, provides a grinding mill for particulate material, including a rotary container having an inner surface, feed means for feeding the particulate material to the container, means rotating the container at a sufficiently high speed that the particulate material forms a layer retained against the inner surface throughout its rotation, and shear inducing means contacting said layer so as to induce shearing in said layer.
In non-vertical mills, the minimum rotational speed at which the particulate material rotates around in contact with the container is known as the xe2x80x9ccritical speed.xe2x80x9d That term is used herein with reference to both vertical and non-vertical mills as referring to the minimum rotational speed at which the particulate material forms a layer retained against the container inner surface throughout its rotation.
The invention also provides a grinding method in which particulate material is fed to a container rotated at above critical speed, so as to form a layer retained against the container throughout its rotation and inducing shear in said layer by shear inducing means contacting the layer.
Preferably, the shear inducing means is mounted inside and rotates relative to the container.
In a first embodiment, the shear inducing means rotates in the direction of rotation of the container, but at a different speed. In a second embodiment, the shear inducing means counterrotates relative to the container.
Alternatively, the shear inducing means can be non-rotational, relying on relative rotation with the container to induce shearing of the material layer.
Preferably also, the mill rotates at least three times, more preferably at least ten times, critical speed.